Thesis: develop self-healing and physical vapor depositions coatings which can be used as coating materials to inhibit microbial-induced corrosion in marine environment.

Background of Research

In the majority engineering resources appliances microbiological impacted corrosion (MIC) is a grave matter [1]. This matter could be further dangerous within marine vessels such as ships and maritime platforms in addition to offshore jetties and rigs [2]. These constructions have to be sheltered against assault of main components in marine setting such as saltwater, temperature influence and biological attack, also known as “Biofouling”. Biofouling is the colonization of submerged surfaces of structures by unwanted organisms such as bacteria, barnacles and algae. Marine biofouling is a long standing and costly problem for the maritime industry as the growth of fouling assemblies on ship hulls, for example, increases drag, reduces maneuverability and increases fuel consumption and greenhouse gas emissions [3-6] resulting in both high economic and environmental costs[7]. Marine fouling, or the settlement and growth of marine organisms on submerged structures is estimated to have a global cost of more than $3 billion annually[8]. Shipping accounts for approximately 90% of world trade and seaborne trade has quadrupled over the past four decades[9, 10]. Overall, MIC is not a new type of corrosion, but most commonly appears in the form of localized corrosion whether pitting or crevice corrosion. MIC initiate, extent or burgeoned due to the presence of microorganisms such as bacteria[11, 12].

Many traditional antifouling systems are paints, which is a comprehensive term covering a variety of materials: enamels, lacquers, varnishes, undercoats, surfacers, primers, sealers, fillers, stoppers and many others. The use of toxic antifoulants on ship hulls has been a historic method of controlling fouling but biocides such as lead, arsenic, mercury and their organic derivatives have been banned due to the environmental risks that they posed. Antifoulants are one of many additives usually incorporated within the topcoat paint of a marine protective coating system[2].
Pseodomonas aeruginosa is a dominant bacterium in marine environments, and one of the aerobic slime former bacteria, which forms a biofilm layer on the steel surface. The chemical reaction of biofilm layer with the steel and the formation of differential aeration cells create conditions on steel, which initiate and accelerate the corrosion process. The generation of these concentration cells is detrimental to the integrity of the oxide layer and enhances the susceptibility of steels to corrosion [13-15]

In the past decade, a novel method that has been achieved for automonic healing micro cracks and mechanical damage is through the use of self-healing polymer coatings[16]. Self-healing coatings are a highly improved category of smart materials where the objective is totally sufficient, passive repair of micro-cracks without the need for detection or any type of foreign intervention [17-21].

Another method of combating biofouling is physical vapor deposition coating (PVD) is one of the various methods that can be used to deposit silicon oxide coating onto a substrate. PVD comprises a variety of vacuum deposition methods and it is a general term used to describe a method that deposit thin films by the condensation of vaporized form onto various substrate surfaces. The basic PVD process fall into two general categories: sputtering and evaporation. The applications of PVD techniques range over a wide variety of applications from decorative, to high temperature superconducting films[22].

1.2    Problem Statement
MIC of immersed structures in marine environments is the result of surface colonization and adhesion processes used by biological organisms. Since the critical bio-interfacial processes resulting in fouling are nano-scale / micro-scale in dimension. It is probably essential that the surface properties of the structures to control biofouling should be on the same length scales. An area of particular interest in recent years is the use of nanotechnology in combating MIC. There is a need to find environmentally friendly coatings to inhibit MIC effectively. Indeed, recent research has shown that self-healing coatings concept based in releasing healing agent when micro-cracks are initiated in the coating surface and silicon oxide based organic and inorganic coatings as well as diamond-like carbon coatings have great potential for use as antifouling with less negative impact on the environment. The new approaches are based on “fouling release” and “contact killing”. The “fouling release” approach does not involve the release of biocides in marine water and thus should be environmentally friendly, while the contact killing approach is favorable and polycationic coatings are used to inhibit MIC through this strategy. However, in order to achieve such a goal the coating should be tailored in such a way its surface properties would have excellent smoothness and corrosion resistance, high hardness, thermal stability and low friction as well as cost effective. Due to their biocide behavior and anticorrosive properties environmentally friendly, silicon oxide and zeolite and polyaniline are good candidates to protect metal surfaces against MIC.

1.3    Purpose of the Research
The purpose of this research is to investigate on environmentally friendly coatings for MIC inhibition applications. First, the research induces an investigation on the mechanisms of MIC behavior of steel in bacteria inoculated medium. This could be useful to enable application of efficient mitigation programs to inhibit MIC of steel. Second, the research induces an investigation on the MIC inhibition properties of self-healing and physical vapor depositions coatings. The output of this research is expected to improve the MIC inhibition properties of coated steels exposed to bacteria inoculated medium. This study is expected to provide the good candidate MIC inhibition coatings with are effective and also have an environmentally friendly nature.

1.4        Objectives of Research
The objectives of the research are as follows:

  1. To develop self-healing and physical vapor depositions coatings which can be used as coating materials to inhibit microbial-induced corrosion in marine environment.
  2. To determine the microstructure and properties of the self-healing and physical vapor depositions coatings on the carbon steel.
  3. To evaluate the performance of self-healing and physical vapor depositions coatings as antifouling coatings

1.5      Scope of Research
    The scope of project is as follows:

  1. Synthesis of the self-healing core materials including zeolite, polyaniline, zeolite-polyaniline composite and their microcapsules through in situ chemical polymerization method and applying them on steel substrates.

 

  1. Deposition of silicon-oxide based coatings on steel substrates with improved properties using physical vapor deposition technique
  2. Characterisation of the synthesised and deposited coatings including hardness, coating adhesion, Field Emission Scanning Electron Microscopy (FESEM), , Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Energy Dispersive Spectroscopy (EDS), X-ray, Electrochemical Tafel Analysis and Electrochemcial Impedance Spectroscopy (EIS).
  3. Biological assays will also be conducted in order to evaluate the antifouling capabilities of the developed coatings against P. aeruginosa bacteria.
  4. Perform immersion test at varying immersion time.
  1. Analysis of samples after immersion test using the standard characterisation equipment.

1.6    Significance of the Research

The basic aim of this research is to provide significant information on the microbial-induced corrosion (MIC) inhibition behavior of self-healing and physical vapor depositions coated steel exposed to bacteria inoculated medium. Thus, the results of this research will benefit the many industries, especially in maritime, oil and gas fields.

CHAPTER 2

LITERATURE REVIEW

2.1    Corrosion

Corrosion is defined as destruction or deterioration of a material, because it is a form of destructive attack of a metal by chemical or electrochemical reaction with its environment. In the most common use of the word, corrosion means a loss of electrons of metals reacting with water and oxygen. In the other way, some of the scientists think that deterioration by physical cause does not belong to corrosion, but is described as erosion, galling, or wear[23]. It is suggested that some of the chemical attack will accompany physical deterioration, for example corrosion – erosion, corrosive wear, or fretting corrosion, including both destruction and deterioration into the concept of corrosion[24].
Corrosion is an electrochemical process in which a current leaves a structure at the anode site, passes through an electrolyte, and reenters the structure at the cathode site as shown in Figure 2.1. For example one small section of a pipeline may be anodic because it is in an environment with low resistivity compared to the rest of the line. Current would leave the pipeline at that anode site, pass through the environment, and reenter the pipeline at a cathode site. Current flows because of a potential difference between the anode and cathode. That is, the anode potential is more negative than the cathode potential, and this difference is the driving force for the corrosion current. The total system—anode, cathode, electrolyte, and metallic connection between anode and cathode (the pipeline in Figure 2.1) is termed a corrosion cell[25].
Figure 2.1: Corrosion of a Pipeline Due to Localized Anode and Cathode (Source: Technical manual, Headquarters Department of The US Army Washington, (1985)[25].

Common structural metals for instance steels are produced from minerals “iron ores”, which require a large amount of energy to be produced like heating and melting, and this large energy makes the metals unstable and wanting to react with the environment to revert back to their original state. Certain environments offer opportunities for these metals to recombine and revert to their lower energy states and some do not. The stored energy in the pure metal while producing from ore is a driving force of corrosion reaction. Metals willing to be in the stable energy level by losing electrons they have received during production processes, establish the main concept of corrosion. In other words, corrosion can be described as “extractive metallurgy in reverse”. Figure 2.2 and 2.3, shows the thermodynamic stability curve of the metal in different types[24]

Figure 2.2: Free energy curve of the metal in different types[24] Figure 2.3:Refining-corrosion cycle[26] In order for corrosion to occur, four components should be met: anode or anodic sites on the surface of the metal where oxidation or corrosion occurs, cathode or cathodic sites on the surface of the metal where reduction occurs, electrolyte in contact with both anode and cathode to provide a path for ionic conduction, and finally electrical connection between anode and cathode to provide a path for the flow of electrons[24].
When corrosion occurs, oxidation and reduction reactions are taking place at the metal surface and the metal lose electrons (i.e., oxidation) or gain electrons (reduction):
Fe → Fe2+ + 2e–       Oxidation (anodic reaction)               (2.1)
Iron atom → Iron ion (ferrous) + electrons
2H+ + 2e– → H2↑    Reduction (Cathodic reaction)              (2.2)
Hydrogen ions + electrons → hydrogen gas
In our societies, water is used for a wide variety of purposes, from supporting life as potable water to performing a multitude of industrial tasks such as heat exchange and waste transport. The impact of water on the integrity of materials is thus an important aspect of system management. Nearly all metallic corrosion processes involve transfer of electronic charge in aqueous solutions. Thus, to understand the electrochemical nature of aqueous corrosion it is necessary understand their electrochemical reactions. Basically all environments like acidic or alkyd are corrosive to a certain degree[27].
Figure 2.4 and figure 2.5 show the electrochemical reactions of iron in acidic and alkyd environments.
Figure 2.4: Schematic of the electrochemical nature of corrosion process in HCl environment.[27]

Figure2.5: Schematic of the electrochemical nature of corrosion process in Alkyd environment[24]

Corrosion appears in different forms and types including: (1) uniform corrosion, (2) galvanic corrosion, (3) inter-granular corrosion, (4) crevice corrosion, (5) pitting corrosion, (6) hydrogen damage, (7) stress corrosion (8) erosion corrosion, as shown in Figure 2.6,  and the most common environments for that are soil, marine, atmospheric moisture ,and bacteria contaminated environment[28].

Figure 2.6: Diagrammatic summary of the various types of corrosion[28].

2.2    Microbiologically induced corrosion and Biofouling

Microbiologically influenced corrosion is one of the most important types of corrosion and is known as MIC. It can appear in the form of pitting or crevice corrosion based on the microbial activities [11]. In marine environment MIC is a part of accelerated low water corrosion (ALWC) [29]and can be observed in all ports across the globe[30-32]. MIC is another way of referring to biofilms and biofilms are an integral part of most MIC cases. It starts and extent are due to the presence of microorganisms and forming biofilm layer [10, 11] The impact of MIC and biofouling appears to affect different industries as shown in Table 2.1 and Table 2.2.

Table 2.1:  Industries known to be affected by MIC {Little, 2008 #190}[32]

Industry Problem areas
Chemical-processing Stainless steel tanks, pipelines, and flanged joints, particularly in welded areas after hydro testing with natural river or well waters
Nuclear power generation Carbon and stainless steel piping and tanks; copper nickel, stainless, brass and aluminium bronze cooling water pipes and tubes, especially during construction hydro test and outage periods
Onshore and offshore oil and gas Mothballed and water flood systems: oil and gas handling systems, particularly in those environments soured by sulfate reducing bacteria (SRB)-produced sulfides
Underground pipeline Water-saturated clay-type soils of near-neutral pH with decaying organic matter and a source of
SRB
Water treatment Heat exchangers and piping
Metal working Increased wear from breakdown
of machining oils and emulsions

Table 2.2: Occurrence of MIC in power plants[32]

System/ component Material Evidence
Fire protection Carbon steel Clogging; through
wall pits at threaded
connections
Heat exchanger Aluminium brass
70/30 CuNi, 90/10 CuNi, Incoloy 800
Pitting; sulfur
deposits present
with pitting
Demineralized water storage tank 316 stainless steel Rust nodules; vertical rust stain at welds
Cooling tower Galvanized steel,  wood Wood rot
Pump impeller Stainless steel Pitting

In general, there are many different types of microorganisms which may cause corrosion of metals and bacteria being the most common one. Bacteria can be classified into aerobic bacteria for instance pseudomonas aeruginosa [30]bacteria which can live and proliferate under aerobic conditions like cooling system[32], and anaerobic bacteria like sulphate reducing bacteria which can live under anaerobe conditions (SRB)[33]. Some of the microorganisms that may cause fouling are listed in Table 3.2.
Table 2.3: Common microorganisms which caused MIC

BACTERIA OXYGEN
REQUIREMENT
METALS ACTION
Desulfovibrio Anaerobic Iron, steel, stainless steel ,aluminium, zinc, copper Reduces sulphate to sulphide ions,
Desulfotomaculum Anaerobic Iron, steel, stainless steel Reduces sulphate to sulphide ions,  forming hydrogen sulphide; forms  sulphide scales
Thiobacillus
ferrooxidans
Aerobic Iron, steel Oxidizes ferrous ions to ferric ions
Gallionella Aerobic Iron, steel, stainless steel Oxidizes ferrous ions to ferric ions
Pseudomonas aeroginusa Aerobic Iron, steel, stainless steel Reduces ferric ions to ferrous ions

2.2.1     Bacteria
Bacteria are small microorganisms that are ubiquitous in environment, especially in marine environments.  Bacteria are generally classified by their morphology and gram strain.  Based on their shapes, bacteria are mostly existed in the forms of sphere, rods and spiral.  Based on their gram strain, bacteria are divided into gram positive and gram-negative types[34].

2.2.1.1 Gram Positive Bacteria

In gram-positive bacteria, the cell wall consists of three layers: 1) cell wall protein 2) cell membrane 3) cytoplasmic membrane[35].  Cell membrane consists mainly of peptidoglycan (15-25 nm thick) and comprising multiple layers of repeating units of two sugar derivatives, N-acetylglucosamine and N-acetylmuramic acid, and a small group of amino acids that is rich in carboxylate groups that are responsible for the net negative charge on the bacteria cell wall [35].
2.2.1.2 Gram Negative Bacteria
In gram-negative bacteria, the cell wall is structurally more complex than gram-positive type.  It has three different layers consisting of 1) outer membrane, 2) periplasm (contains peptidoglycan layer), and 3) inner membrane.  The peptidoglycan layer is about 3 nm thick and does not contain secondary polymers [35].  The outer membrane is constructed of phospholipid and lipopolysaccharide (LPS) which is highly anionic.  LPS consists of O-polysaccharide, the core polysaccharide, and lipid A.  In gram-negative cells, LPS has the major role in metal binding because of its high concentration of phosphate and carboxyl groups [35].
Generally, the cell wall of bacteria (either gram positive or gram negative) is electrically negative [35].  This negative charge contributes to their tendency to adhere to the metallic substrate.  The bacteria attachment to the metal substrate is the initial stage of biofilm formation [36].

2.2.2        Biofilm

The biofilm is a mass of microorganisms and their extracellular polymeric substances (EPS) like proteins and lipids. EPS products provide enzymatic interaction, exchange of nutrients and protection against environmental stress[37]. Using ferrous metal as an example, a schematic model of corrosion reactions involving EPS-bound metal ions in biofilms is depicted in figure 2.8
Figure 2.8: Schematic representation of the cathodic depolarisation reaction of a ferrous material in the presence of an oxygenated biofilm, owing to Fe3+ binding by EPS. (a) Fe3+, obtained as a result of, for example, oxidation of anodically produced Fe2+, is bound by EPS and the Fe3+–EPS complex is deposited on the metal surface. (b)  Electrons are transferred directly from the zero valent Fe to EPS-bound Fe3+, reducing it to Fe2+. In the presence of oxygen, acting as terminal electron acceptor, Fe2+ in EPS is reoxidised to Fe3+  [37] The process of fouling is usually based on the key growth stages which include 1) an initial accumulation of adsorbed organic molecules on the metal surface, 2) the settlement and growth of pioneering bacteria creating a biofilm matrix through formation of extracellular polymeric substances, 3) the subsequent succession of micro and macro-foulersand 4) detachment of bacteria cells from the biofilm outer surface[2] as shown in Figure 2.9

Figure 2.9: Schematic of biofilm formation stages[2] The macro-foulants include barnacles which attach on the biofilm changing its surface characteristics[10]. Antifouling systems are required wherever unwanted growth of biological organisms occurs. This is often in most saline aqueous phase environments; hence applications include medical, freshwater and marine systems. Marine engineered systems have been categorized into seven key types of submerged structures of which ship hulls account for 24% of the total objects fouled[38]. The fouling of ship hulls is often prolific as vessels move between a diverse range of environments and remain constantly in the most productive region, the photic zone, of the water column. Although coatings and biocides are used for hull protection, they can fail due to the build-up of inorganic salts, exopolymeric secretions, and the calcium carbonate skeletal structures that form the fouling organisms[10] .
2.2.3     Differential Aeration Cell
Generally, when the biofilm layer is formed on the steel substrate, it has a heterogeneous state and creates two different areas called anode and cathode.  The anode located beneath biofilm layer is anoxic due to lack of oxygen and the cathode located at the surface of steel, which is enriched with oxygen.  The anode and cathode areas activate differential aeration cells resulting in corrosion acceleration of steel substrate [39].  Due to heterogeneous nature of biofilm, the most probable type of corrosion is localized pitting or crevice (Figure 2.2).
Figure 2.2 Schematic of pitting on the metal substrate in presence of biofilm [39] 2.2.4     Corrosion Causing Bacteria  
Generally, the bacteria within the biofilm could be alive and activate based on their oxygen requirement.  In this regard, the bacteria are divided into a) aerobic such as P.aeruginosa and b) anaerobic such as Desulfovibro sp.  Aerobic bacteria can live and proliferate in oxygen containing environment, and anaerobic bacteria can live without need of oxygen.  It is reported that both aerobic and anaerobic bacteria within their biofilm layer cause steel destruction.  Thus, based on their metabolism, corrosion causing bacteria could be divided into aerobe and anaerobe.  They are further subdivided into different types such as iron oxidiser, iron reducer, slime former [40, 41].

2.3    Mechanisms of Microbial-Induced Corrosion of Steels  
Generally, to understand the MIC mechanisms of steels caused by bacteria, two main issues have to be considered: a) the metabolism of bacteria during growth and b) the extracellular polymeric substances (EPS) produced by biofilm and its chemical reaction with steel [42, 43].  It is important to note that talking about bacteria means the bacteria in biofilm layer.  In the first mechanism, based on their metabolic activity, bacteria are divided into aerobic and anaerobic groups and subdivided into iron oxidizing bacteria (IOB), sulfur oxidizing bacteria (SOB), slime forms, sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB).  Due to their metabolic activity, bacteria in the biofilm make some changes on the steel surface which induce corrosion damage [44-47]. In the second mechanism, EPS which is secreted during biofilm formation could also influence on the corrosion behavior of steel.
The chemical reaction of EPS and steel substrate could induce corrosion damages [48].  In the following sections, the MIC mechanism caused by aerobic and anaerobic bacteria is discussed through the metabolism of bacteria and the EPS-metal interaction.  It is important to note that these mechanisms might occur simultaneously happen to induce the corrosion damage of steels.  It is generally known that, the differential aeration cell is the general MIC mechanism for almost all corrosion causing bacteria since they form a biofilm layer on the steel substrate.
2.3.1 Mechanisms of Microbial-Induced Corrosion through Anaerobic Bacteria
Generally, the main anaerobic bacteria which induce corrosion damages on steels are sulphate reducing bacteria (SRB) [46, 49] and Iron reducing bacteria (IRB) [45].

2.3.1.1 Sulphate Reducing Bacteria
Sulfate-reducing bacteria (SRB) are widespread in nature and influence the geochemical cycling of carbon, sulfur and metals in marine and aquatic sediments and terrestrial subsurface environments[50].sulfate-reducing bacteria can create problems when metal structures are exposed to sulfate-containing water: Interaction of water and metal creates a layer of molecular hydrogen on the metal surface; sulfate-reducing bacteria then oxidize the hydrogen while creating hydrogen sulfide, which contributes to biofouling[51]. SRB can change the composition and crystal structure of steel which the former protective layer becomes loose, porous and easy to be desquamated [52].

Figure 2.7: Sulfate-reducing bacteria (Anaerobic Bacteria)[53] Sulphate reducing bacteria (SRB) in their biofilm state is responsible for pitting damages of steels in aquatic environment under anoxic conditions.  SRBs produce hydrogen sulphide (H2S) during their metabolism.  This H2S could react with iron and forms iron sulfide (FeS) deposits on the steel substrate; if SRBs are growing on steel surfaces, their biofilms are likely to promote the formation of pits beneath sulphide deposits [54].
Figure 2.3, shows the schematic of corrosion damage in presence of SRB. Generally, the cathodic depolarization mechanism is believed to have a secondary role on pitting corrosion of iron in presence of SRB.  It is found that the main pitting causing factor is the formation of galvanic cell between the corrosion products (FeS) and the steel substrate [54].
Figure 2.3     Cathodic depolarization of iron caused by SRB [54] 2.3.1.2 Iron Reducing Bacteria
Iron reducing bacteria (IRB) are the bacteria with the ability to reduce the insoluble Fe (III) to soluble Fe (II) which is related to their growth and metabolism.  These bacteria, have a major role on availability of iron ions through solubilization of insoluble iron oxides.  Thus, they are able to induce the corrosion of steels through the reduction and removal of passive oxide films from the metal surface [45]. Shewanella oneidensis is known as one of the important iron reducing bacteria, which cause corrosion damage of the steels [55].
2.3.2     Microbial-Induced Corrosion Mechanism Caused by Aerobic Bacteria
Based on their metabolic activity aerobic bacteria, are divided into a) metal oxidizing bacteria and b) slime formers.  The metal oxidizing bacteria generally lead to production of aggressive compounds, which induce corrosion damage[56].  The slime forming bacteria could also induce corrosion through formation of differential aeration cells [13, 14].
2.3.2.1 Metal Oxidising Bacteria
The main aerobic corrosion causing bacteria are sulphur oxidizing bacteria (SOB) and iron oxidizing bacteria (IOB).  SOB bacteria due to their metabolic activity can release aggressive products, such as organic (acetic, succinic) or inorganic acid (sulphuric) which deteriorates steels integrity and causes severe corrosion failures.  An example of SOB is Thiobacillus that can form sulphuric acid, which is a strong corrosive agent that causes severe damage on steels as well concrete [40].
IOB are ubiquitous and have been identified in numerous environments. IOB can oxidise Fe (II) to Fe (III) which is precipitated on the steel surface due to it metabolic activity and conserve energy from this process and convert inorganic carbon, in the form of carbon dioxide (CO2), in the biofilm[57].  The tubercles can lead to crevice attack, and can also provide a suitable environment for the anaerobic bacteria in the region beneath the tubercle (Figure 2.4).  Different iron oxidizing bacteria are involved in corrosion of steels, such as Gallionella, Leptothrix, and Siderocapsa species[58].
Figure 2.4 Schematic of corrosion damage in presence of metal-depsiting bacteria[58] 2.3.2.2 Slime Former Bacteria  
Another group of bacteria that can cause corrosion of steels are slime former bacteria, which form a dense slime layer on the substrate.  The slime growth on the steel surface could create differential aeration cells.  Presence of biofilm layer on the steel causes the formation of two areas: a) cathode which is placed at the steel surface and rich in oxygen and b) anode, which is located beneath the biofilm layer and has less oxygen.  The differential aeration cells thus formed localized pitting or crevice corrosion[39].  Moreover, the slime exudate from the bacteria is generally acidic, which also deteriorate   the steel substrate.

  1. aeruginosa is known as one of the important types of slime former aerobic bacteria, which accelerate the corrosion rate due to the formation of differential aeration cells in presence of its biofilm layer.  It is also found that P. aeruginosa has the ability to reduce ferric to ferrous iron, exposing steel to further oxidation as ferrous iron compounds are more soluble and the protective ferric iron layer is solubilized by this process [40].

The general MIC mechanism caused by different types of bacteria basically based on their metabolism activity.  Another important mechanism of MIC is related to EPS secreted from the biofilm layer.  The chemical reaction of EPS-metal substrate also induces the corrosion damage of the steel substrate [43].

2.3.3 Microbial-Induced Corrosion Mechanism through EPS-Metal Interaction
The chemical reaction of EPS of biofilm and the steel substrate has an important role to cause MIC.  The EPS comprised of macromolecules such as proteins, polysaccharides, uronic acid, nucleic acids (DNA) and lipids (fatty acids) [59].  These molecules have anionic functional groups (carboxyl, phosphate, sulfate, glycerate, pyruvate and succinate) with metal binding capacity.  Due to the presence of anionic functional groups, EPS could also bind to metals.  The multivalent metal ions such as Mg2+ and Fe3+ have a good affinity for binding to anionic groups of EPS.  The presence of metal ions in different oxidation states in EPS could result in considerable shifts in the standard redox potential. For example, Fe (III/II) redox potential varies significantly with different ligands (in EPS) (from +1.2 V to -0.4 V).  EPS bound metal ions can, therefore, act as electron ‘shuttles’ and open up novel redox reaction pathways in the EPS/metal system, such as direct electron transfer from the metal.  In the presence of a suitable electron acceptor (e.g. oxygen in oxic systems or nitrate under anaerobic conditions), such redox pathways would lead to depolarization of cathode, and thus enhance the corrosion process[60].  Although the presence of metal ions within the biofilm matrix is related to MIC, the role of EPS-bound metal ions in direct electron transfer from the base metal to a suitable electron acceptor is the major cause of corrosion[60].
2.4     Microbial-Induced Corrosion Caused by Pseudomonas aeruginosa Bacteria
2.4.1     Pseudomonas aeruginosa  

Pseudomonas aeruginosa, the most prevalent in industrial water and seawater, have been found to be involved in the corrosion process of mild steel, stainless steel and aluminum alloys in marine habitats[50, 52]. It is a gram-negative, rod-shaped bacterium that has an incredible nutritional versatility. It forms a heterogeneous biofilm layer on the metal surface and causes the formation of differential aeration cells which can be a reason of forming biofilms. As shown in Figure 2.7, this bacterium is a rod shaped about 1-5 µm long and 0.5-1.0 µm wide.

Figure 2.6: Pseudomonas aeruginosa (Aerobic Bacteria)[61]

  1. aeruginosa is an aerobic slime former bacterium that is dominant in marine environment.  This bacterium has a rod shape with around 1.0–2.5 µm long and 0.4– 0.6 µm in diameter [62]. This type of bacterium in its biofilm state could be detrimental for the steels and cause severe corrosion damages. It forms a heterogeneous biofilm layer on the steel surface and causes the formation of differential aeration cells, which induce localised corrosion[63]. Various Pseudomonas isolates have been also implicated in the reduction of ferric to ferrous iron, exposing steel to further oxidation as ferrous iron compounds are more soluble and the protective ferric iron layer is solubilized by this process [40].  Generally, the main MIC mechanisms caused by P. aeruginosa biofilm could be divided into two: a) differential aeration cell due to biofilm formation which induce corrosion damages b) the chemically reaction between EPS and steel substrate which the EPS-metal biomineral act as corrosion inducer c) role of siderophore produced by P.aeruginosa in iron reduction.

2.4.1.1 Differential Aeration Cell Caused by Pseudomonas aeruginosa Biofilm Layer

  1. aeruginosa is a slime former bacterium which tends to form a biofilm layer on steel surface.  Due to presence of heterogeneous biofilm layer on the steel, different areas of aeration cells are created.  The anode is located beneath the biofilm layer and lack of oxygen and cathode is placed at the rest of steel substrate which is enriched of oxygen.  The difference in oxygen concentration between anode and cathode lead to activation of electrochemical cells which induce the corrosion damage in the form of pitting or crevices [39].

2.4.1.2 The Interaction of EPS of Pseudomonas aeruginosa with Steel
Generally, P. aeruginosa during biofilm formation, excrete acidic EPS, which contains exopolysaccharide (alginate) [64].  Alginate contains anionic carboxylic groups with metal binding ability.  Generally, EPS tends to react with Fe, to gain energy for bacteria growth through releasing proton from the carboxylic groups in EPS.  The dominant cation of Fe in the environment is important for EPS-Fe (III) interaction.  For example, in the seawater environment, one of the dominant ion of Fe (III) is in the form of Fe (OH) 2+ this ion interacts with the carboxyl group present in EPS.  By interaction of Fe (III) with EPS, the acidity in the environment would be increased.  This can be an indication that protons from EPS are released into environment, through deprotonation of carboxylic groups [65].
Thus by dissociation of carboxylate groups in EPS, RCOOH converts to RCOO which these anions would be responsible for the interaction with the ferric ion.
The proposed chemical reaction of EPS and Fe (III) is shown in reaction 2.4 [65].
RCOO
2RCOOH + Fe (OH) 2+                        Fe (OH)    + 2H+                  (2.4)
RCOO
According to reaction (2.4), the interaction mechanism assumes that each iron atom is associated with two carboxylate groups, COO-, which combined forming an oxalate group,C2O4 2-, therefore it is likely the presence of iron oxalates in the EPS loaded with Fe (III).  Thus, the EPS of P. aeruginosa could interact with ferric ion on the steel substrate (Fe (OH) 2+) through their carboxylic groups (RCOOH) result in formation of Fe (OH) (C2O4) × 2 (H2O) as the corrosion product.  This mineral product could act as a catalyser for cathodic depolarization, which induces the corrosion damages on the steel substrate.
2.4.1.3 Role of Siderophore Produced by Pseudomonas aeruginosa in Iron Reduction
Generally, iron is an essential nutrient for bacteria such as P. aeruginosa.  The iron is mostly oxidized from soluble Fe+2 to Fe+3 and insoluble ferric oxyhydroxide in presence of oxygen and neutral pH.  The insoluble ferric oxyhydroxide is not useful for the P. aeruginosa bacteria, since the bioavailability of iron is just around 10-9-1010 M.  In such condition, that the concentration of iron is too low, the P. aeruginosa produce a chelator known as siderophores.  Siderophores are low molecular weight compounds with high affinity for iron, which solubilize ferric ions and transport these ions into the bacteria cell for growth and activity of bacteria.  The siderophores of P. aeruginosa are pyochelin (PCH) and pyoverdin (PVD) [66] (Figure 2.5).  These sidophores could uptake the iron from the steel substrate for growth and activity of P. aeruginosa [67].  The sidophore-iron interaction and uptaking the iron from the steel substrate might enhance the corrosion process of steel.
Figure 2.5 Chemical structure of pyochelin the siderophor of P. aeruginosa [67] 2.4.2     Effects of Microbial-Induced Corrosion of Steels in Presence of Pseudomonas aeruginosa
Many researches were done to investigate the effect of P. aeruginosa biofilm on MIC process of steels [13, 14, 63].  In 1993, J. Morales et al have investigated on localized pitting of 304 stainless steels in presence of P. aeruginosa in the neutral buffered solutions.  Their results showed that stainless steel is susceptible to pitting corrosion due to activity of P. aeruginosa in the biofilm state through two different ways: a) by formation of differential aeration cell which induces pitting corrosion under biofilm layer, and b) by reducing the thickness of passive oxide layer due to metabolites production [63].  Yuan et al [62] have studied microbiologically influenced corrosion of 304 stainless steels by aerobic Pseudomonas NCIMB 2021 biofilm [62].  Figure 2.6 shows the depth of pits on the steel substrate after different exposure times in bacteria incubated medium.  As shown in Figure 2.6 (a), the shallow pit with a depth 150 nm appeared on the steel substrate after 14 days exposed in bacteria incubated medium indicate the initiation of micro-pits.  By extending the exposure time, the width and depth of pits increased.  After 28-day exposure time, the pits with a depth 440 nm and after 49 days, the pits with depth of 770 nm were observed on the steel substrates (Figures 2.6 (b) and (c)).  The increase in the depth of pits on the steel substrate by extending the exposure time in bacteria incubated medium reveal the aggressive role of the bacterium in inducing the corrosion damage [62].
Figure 2.6 Atomic force microscopy images of the presence of pits on the corroded surfaces of the stainless steel 304 coupon after different exposure times: (a) 14 days; (b) 28 days; (c) 49 days [62] Yuan et al., [47] have studied the corrosion reaction of stainless steel in artificial sea-water in the presence of Pseudomonas bacteria.  By doing electrochemical analysis such as Tafel plots, they inferred that the corrosion rate of steel samples in bacteria contained solution increased by extending the exposure time.  In contrast, the corrosion rate of the steel samples in the sterile medium was constant due to the protective role of chromium oxide layer naturally formed on the stainless steel substrate [47].
Another interesting result that the authors observed was the synergistic effect of biofilm layer and chloride ions on occurrence of severe pitting corrosion on the steel specimens.  Figure 2.7 shows the EDS analysis of pit areas on the sample immersed in bacteria contained simulated seawater.  The EDS showed that the pits are enriched with carbon, oxygen, and chlorine due to presence of bacterial cells, their EPS, and chloride Cl- in the pitted areas.  Also it was observed that the amount of metallic elements such as iron, chromium, nickel, and manganese have decreased in the pitted areas, this could be related to the occurrence of localized corrosion in the presence of aggressive Cl- and the colonizing bacteria.  The formation of heterogeneous biofilm layer on steel and presence of aggressive chloride ion in the medium has an important role for initiation of pittings.  There are three prerequisites for occurrence of pits on the metal substrate through Cl- ions, including (i) the presence of Cl- ion in the medium, ii) the existence of potential difference between the anodic area (the pit area) and cathodic area (the rest surface on the metal), and (iii) the reaction temperature must exceed a critical temperature.  The heterogeneous P. aeruginosa biofilm layer creates a potential difference between the anodic (pit) area and cathodic (metal surface) areas through formation of differential aeration cells lead to weaken and breakdown of the oxide layer. Thus, it could be inferred that both Pseudomonas biofilms and Cl- ions have an aggressive role in occurrence of pitting corrosion on steel [47].
Figure 2.7 SEM images and EDX spectra of pit are as formed on the 304 S coupon surfaces in presence of Pseudomonas bacteria after (a) 14 days and (b) 35 days[47] Yuan et al. [62], investigated the role of Pseudomonas NCIMB 2021 biofilm layer on pitting corrosion of 304 stainless steels [62].  Figure 2.8, shows the formation and development of biofilm layer on the steel substrate after different exposure times.  As shown in Figure 2.8 (a), some bacteria cells are distributed on the steel substrate after 3-days exposure to bacteria incubated medium indicates the initial attachment of Pseudomonas bacterium on the coupon surface which is important in further biofilm formation.  Prolong exposure time to 14 days, the bacteria cells tend to secrete polymeric substances (EPS) to improve their binding to steel and form a biofilm layer (Figure 2.8(b)).  By extending the exposure time to 42 days, the more heterogeneous and patchy biofilm layer is formed on the steel substrate [62].

Further exposure from 3 days to 42 days the values of the arithmetic mean roughness, Ra, increase noticeably from initially 48 nm to 246 nm with exposure time, indicative of the increase in heterogeneity of the biofilm on the steel coupon surface (Figures 2.8 (a–c)).  The increase in thickness, and heterogeneity of biofilm layer with time, is detrimental to the oxide layer on the steel surface, since they can give rise to local differences in metabolic products, pH, or dissolved oxygen (i.e., differential aeration cells), all resulting to its deterioration and forming of pitting corrosion [62].
Figure 2.8 Atomic force microscopy images of biofilm layer formed on 304 SS substrates after (a) 3 days, (c) 14 days, and (d) 42 days exposed in Pseudomonas contain medium [62] Figure 2.9, shows the pitting corrosion of two stainless steel substrate exposed to bacteria contained solution in different exposure times.  The two-dimensional images together with the sectional analysis graphs are shown in each group of AFM images.  By increasing the exposure time from 21 days to 42 days, the depth of pits is increased from 320 nm to 500 nm.  By extending the exposure time, the pits become deeper and wider (Figures 2.9 (a) and (b)); this is due to formation of more heterogeneous biofilm layer and also aggressive role of chloride ions [62].
Figure 2.9     Atomic force microscopy images of pits occurred on 304 SS substrates after (a) 21 days and (b) 42 days of exposure in Pseudomonas incubated medium [62] Hamzah et al [13] investigated on the role of P. aeruginosa bacteria on corrosion behavior of 304 stainless steels in nutrient-rich simulated seawater [13].  As shown in Figure 2.10, the FESEM and AFM results depict the formation of biofilm layer on the stainless steel substrate and severe pitting damages under the biofilm layer.  The synergistic aggressive role of chloride ions (Cl-) and the P. aeruginosa biofilm was found to be responsible for severe pitting corrosion of stainless steel [13].  The aggressive role of Cl- ions within the medium to breakdown of the oxide layer on steel substrate and further corrosion acceleration could be described by the following reactions.
Fe2+ + 2H2O + 2Cl-           Fe (OH)2 + 2HCl                     (2.5)
Fe (OH)2 + 3Cl-      FeCl3 + 2OH-                          (2.6)
FeCl3+3H2O           Fe (OH)3 + 3HCl                          (2.7)
The interaction of Cl- ions with the hydroxide layer leads to the formation of a soluble FeCl3 product, and FeCl3 is further hydrolysed to produce a very porous Fe (OH3).  The Fe (OH3) product is not stable and could not protect the steel against corrosion [13].  The aggressive role of biofilm layer to induce corrosion damage can be is due to the biofilm formed on the steel becomes larger, thicker and more heterogeneous with exposure time.  Patchiness and the heterogeneous nature of the biofilm generate a condition on the steel surface promoting local differences in pH, corrosion products and differential aeration cells all induce corrosion damage on steel.  Thus, the synergistic role of aggressive Cl- ions and biofilm layer could induce pitting corrosion on steel [13].

Figure 2.10 (a) SEM image of P. aeruginosa biofilm layer formed on 304 stainless steel substrate after 21 days of exposure in bacteria inoculated NRSS media (b) AFM image of pitting damage after 49 days of exposure in bacteria inoculated NRSS medium [13].

2.5    Microbial-Induced Corrosion Inhibition Methods
To overcome MIC, different methods such as biocide treatment, cathodic protection and coatings have been used [2, 7, 68-70].  Coatings are widely used because of their ease of application, effectiveness, and low cost [2, 7].

2.5.1     Antibacterial Coatings  

Because the initial stage of MIC involve bacterial attachment and biofilm formation on the steel substrate, an antibacterial coating can be used at this stage to effectively inhibit MIC.  There are three main strategies to develop antibacterial coatings: a) biocidal leaching, b) adhesion resistance, and c) contact killing shown in Figure 2.11 [34].  Among these strategies, adhesion-resistance and contact-killing strategies are environmentally friendly because they do not cause the leaching of biocidal agents into the environment.

Figure 2.11     Three main strategies to design antibacterial surface [34]

2.5.1.1 Biocide-Leaching Strategy
Biocide leaching is the traditional strategy for antibacterial and antifouling coatings.  Traditionally, biocidal agents based on metals such as tin, copper and zinc were used in the coating to inhibit biofouling.  When these agents contact water, the metal ions are released into the environment, causing bacterial death.  Biocides kill bacteria and other microorganisms, making them environmentally toxic and hazardous to human health [71, 72].
Biocidal agents can leach from coating in several ways.  In the conventional mode, the biocide leaches freely from the polymer matrix, and there is no control on the speed of its release.  Another mode involves controlled release of the biocide from the coating.  In the controlled release mode, biocide agents bond to the polymeric matrix, which then by degradation of polymer; the biocide is released into the environment and exerts biocidal activity.  Therefore, biocide leaching is controlled, and the life of the coating is higher compared to conventional coating [73].  Irrespective of their mode of release, biocides can also kill microorganisms other than bacteria; therefore, these agents are toxic to the environment [74].
Silver is one of the most commonly used antibacterial materials that inhibits bacterial growth and biofilm formation[75].  The biocidal behavior of silver is related to its interaction with thiol groups in the bacterial cell and deactivation of cellular enzymes and Deoxyribonucleic acid (DNA), leading to bacterial death [76, 77].  Although silver has good biocidal properties, it is toxic to several microorganisms [78], causes severe skin diseases, and is harmful to the human body [79].

Traditionally, Tributyltin (TBT) coatings were extensively used for antifouling applications because of their effectiveness; however, their use was prohibited in 2003 because of their toxic effects in seawater.  Subsequently, copper/zinc-based coatings were used for antifouling purposes; however, these coatings also exhibited toxic effects and their use was banned.  Booster biocides were used to improve the antifouling function of copper-based coating; however, these biocides are also toxic [2, 10, 80].

Recently, natural non-toxic biocides produced by some marine microorganisms have been considered for antifouling coatings.  Marine microorganisms produce specific products (secondary metabolites) to prevent colonization by other organisms.  Although natural biocide compounds are environmentally friendly, these compounds have not been commercialized because they are expensive [10, 81].  Enzymes are another type of non-toxic biocides.  Enzymes are catalytically active proteins and are omnipresent in nature.  Enzymes are incorporated into coating for antifouling applications and they effectively inhibit bacterial growth on substrates [82-84].

2.5.1.2 Adhesion-Resistance Strategy  
The adhesion-resistance strategy is based on surface properties of the coating that prevent the adhesion of bacteria.  This is a biopassive strategy because the surface coating inhibits bacterial attachment without killing the bacteria [85].  Bacterial colonization on the steel substrate occurs in two main ways: (a) initial attachment of bacterial cells to the surface, which is rapid and reversible and (b) biofilm formation; the bacteria secrete polymeric substances to improve adhesion to the substrate (Figure
2.12) [34].
Figure 2.12 Schematic of bacterial adhesion and biofilm formation on the surface [34].
Adhesion-resistance coating mainly prevents bacterial attachment in the first stage of bacterial colonization.  Adhesion-resistance coatings include (a) hydrophilic, (b) hydrophobic, (c) amphiphilic and (d) superhydrophobic coatings.

(a) Hydrophilic Coatings
Hydrophilic polymers such as polyethylene glycol (PEG) [86], polyethylene oxide (PEO) [87], zwitterions [88], and polysaccharides [89] in their well-hydrated form are used to reduce bacterial attachment to the surface; these polymers form an interfacial layer that prevents direct contact between bacteria and the surface [85, 90]. PEG is used in hydrogel coating for antifouling applications; these studies revealed that PEG reduced bacterial attachment on the coated substrate [91, 92].  The disadvantage of hydrophilic antifouling polymers such as PEG is that they degrade over time and therefore lack sufficient stability to inhibit bacterial attachment.
Furthermore, it is important to completely cover the substrate surface with the hydrophilic coating to effectively prevent bacterial adhesion. The presence of defects in the coating will allow bacterial colonization and biofilm formation [85, 93].

(b) Hydrophobic Coating
Hydrophobic coatings are a type of adhesion resistance coating with low surface energy.  In hydrophobic coatings, bacteria are easily released from the surface, and the coating is immune to bacterial attachment and biofilm formation [90, 94].  These coatings are based on fluoropolymers and silicone-based polymers and are also called fouling-release coatings; they minimize the adhesion of bacteria to material surfaces, leading to the easy removal of bacteria [90, 95, 96].  Several factors such as surface energy, surface roughness and elastic modulus influence the antifouling properties of the coating.  Reduction in the surface energy and elastic modulus leads to improvement in the fouling-release characteristics of the coating. Increased surface roughness might also improve the fouling-release properties of coating [90, 97, 98].
Increasing the amount of SiO2 nanoparticles lowers the (γ.E) ½ (γ=surface energy and E=elastic modulus) value and improve the antifouling behavior of coating [99].  It is reported that the attachment of microorganisms is almost completely prevented on the coated substrate with the lowest (γ.E) ½ value.  By increasing the (γ.E) ½ value, the number of microorganisms is increased, and the substrate is more susceptible to biofouling.  Because increasing the amount of SiO2 nanoparticles lowers the (γ.E) ½ value, addition of SiO2 could improve the antifouling behavior of coating [99].
Siloxane has been also used in matrix coating for antifouling applications.  The siloxane moiety improved the antifouling properties of coating through lowering the surface energy of coating [90].  Nano SiO2 and titanium oxide powder has been used for antifouling purposes.  The results confirmed the improvement in antifouling properties of coating due to fouling release properties of Nano SiO2 and titanium oxide powder [100-103].
(c) Amphiphilic Coating
Another type of adhesion resistance coating is based on amphiphilic coatings. In these coatings, the hydrophobic component is combined with hydrophilic component to effectively inhibit bacteria attachment on the surface [104].  The protein resistance properties of hydrophilic component with fouling release properties of hydrophobic component could synergistically improve the antibacterial behavior of the coating.
The combination of hydrophobic fluorinated moieties with hydrophilic PEG is extensively studied, and this combination yields good antifouling properties.  The hydrophilic component lowers the adhesion of proteins and microorganisms due to low interfacial tension of the PEGylated component with water.  The fluorinated hydrophobic component can inhibit bacterial attachment by its fouling-release activity [105-108].  Hydrophobic perfluoropolyethers (PFPEs) are cross-linked with a series of hydrophilic polyethylene glycols (PEGs) for antifouling applications.  PFPE is used to reduce the surface energy of coating for fouling-release properties, and PEG is added to weaken adhesion of the fouling to the coating [107, 108].
Amphiphilic coatings are also used to inhibit MIC [109, 110].  Hydroxamic acid amphiphiles were used as self-assembled monolayers (SAM) to protect the metal substrate exposed to cooling water medium contaminated with bacteria.  These studies revealed that fewer bacterial colonies were attached to the coating.  The coating also lowered the corrosion rate, indicating the anticorrosive behavior of amphiphilic coating [109].
(d) Superhydrophobic Coatings
Superhydrophobic coatings are another type of adhesion resistance coating that has recently been examined for MIC inhibition and antifouling [111, 112].  These coatings have a water-contact angle of more than 150 °C with high surface roughness and low surface energy.  Bacteria are also removed from the superhydrophobic surface due to reduced protein adsorption and a trapped air layer between the surface and the bacteria [113].
One type of superhydrophobic surface was developed on a metal substrate by chemical pretreatment followed by dip coating in myristic acid.  This superhydrophobic substrate was exposed to seawater medium to examine its anticorrosion and antifouling properties [114].  It was found that, the bare metal substrate was covered with a biofilm layer.  However, the surface of the superhydrophobic substrate contained almost no bacterial cells.  These results reveal that the superhydrophobic coating effectively inhibited bacterial attachment.  Corrosion test results revealed the good anticorrosive behavior of this coating against aggressive ions.  Therefore, superhydrophobic coating enhances both the antifouling and anticorrosive properties of coating [114].
Another type of superhydrophobic substrate was fabricated by dip coating of an anodized Al substrate in myristic acid.  It is revealed lowered bacterial attachment to the superhydrophobic substrate and decreased corrosion current density. Because of their anticorrosive and antibacterial properties, superhydrophobic substrates display MIC-inhibition properties [115].  However, superhydrophobic coatings lack long-term stability, and after long-term exposure, bacterial attachment is not inhibited, and a biofilm layer is easily formed on the coating [113].

2.5.1.3 Contact-Killing Strategy
The contact-killing strategy involves the biochemically-induced death of bacteria in contact with the surface.  In this strategy, positively charged compounds are immobilized on the surface, which electrostatically interact with negatively charged bacterial cell walls, resulting in bacterial cell disruption and death [116].
Some compounds with positively charged groups are quaternary ammonium salts [117], guanidine polymers [118], phosphonium salts [119], chitosan [120], and peptides [121].  Recently, another group of polymers known as intrinsically conductive polymers has displayed biocidal properties [122].  Positively charged nitro-groups are responsible for biocidal behavior [122].  The cationic polymers are immobilized on the steel substrate and subsequently quaternized with different chemicals such as alkyl halides to increase the concentration of positively charged groups, which increases their biocidal properties.  The good mobility of cationic agents is also important for the biocidal property of the polymer.  Therefore, cationic agents are generally immobilized using a spacer such as polyethylene glycol (PEG) or poly (2-methyl-2oxazoline) (PMOX) to ensure that the mobility of group is retained [123].
Non-leaching biocidal coatings containing cationic groups have gained attention in antibacterial surface design and MIC inhibition because they (a) are not toxic to the environment because only the bacteria in contact with the coating are killed, and other microorganisms are immune, (b) are effective against both Grampositive and Gram-negative bacteria, (c) display biocidal properties for a long time, and (d) The bacteria cannot resist these coatings [123-126].
2.5.2     Bi-functional Antibacterial Strategy
To further enhance the antibacterial properties of coating different antibacterial surface design such as biocide-leaching, adhesion-resistance and contact-killing strategies, could be combined together.
2.5.2.1 Biocide Leaching-Contact Killing
The combined biocide leaching-contact killing strategy has been used to inhibit the bacterial attachment and biofilm formation.  The quaternary ammonium salts (QASs) were immobilized on the substrate and the silver nanoparticles are embedded in the coating.  The coated substrate was evaluated for their antibacterial properties.  The results showed the superior biocide properties of QAS-Ag NPs coating in compare of the merely QASs or Ag NPs contained coating within the time.  The synergistically biocide behavior of Ag, NPs, and contact killing nature of QASs on the substrate has cause high biocide behavior of the coating [127].
In another study [128], the polycationic poly (4-vinylpyridine)-co-poly (4vinyl-N-hexylpyridinium bromide) (NPVP) contain silver bromide nanoparticles are coated on the substrate for antibacterial applications.  The coated substrate was exposed to the bacteria inoculated medium for various times.  The results showed that for the 21% NPVP coated substrate exposed to bacteria inoculated medium, there are some bacteria cells attached on the substrate after 24h of exposure time.  After 48h the whole surface of 21% NPVP coated substrate is covered with biofilm layer.  This indicates the insufficient biocide behavior of 21% NPVP coated substrate which cannot avoid the biofilm formation.  The 21% NPVP coating could initially kill the bacteria through contact killing strategy which the positively charged nitro groups interact with bacteria cell membrane lead to death of bacteria.  However the dead bacteria cells remained on the coated substrate and the positively charge groups could not interact with the viable bacteria cells.  Thus, the biocide property of 21% NPVP coating was decreased and biofilm layer was formed on the substrate.  In the case of AgBr/NPVP coated substrate there is almost no bacteria cells attached on the surface after 24h of exposure.  After 72h, there are just few bacteria cells adhered on the surface without formation of biofilm layer.  This indicates the effective biocide behavior of AgBr/NPVP coating to inhibit bacteria attachment and biofilm formation.  The combined biocide properties of Ag ions and NPVP could improve the antibacterial behavior of coating [128].
2.5.2.2 Adhesion Resistance-Contact Killing
The combined adhesion resistance-contact killing strategy has been used for antibacterial coating.  Polysiloxane coating was functionalized with QASs for antifouling applications, and the QAS-polysilxoane coating has superior antifouling behavior.  This might result from the fouling-release properties of polysiloxane coatings combined with the biocide behavior of QASs, which kill bacteria through electrostatic interaction [129].  Poly (dimethylsiloxane) (PDMS) coating has been chemically functionalized with quaternary ammonium salt (QAS) moieties for antifouling applications, showing that the presence of QAS effectively improved the antifouling properties of coating through contact killing [117].  A PEG-cationic polycarbonate coating was applied to a substrate for antifouling [130].  The coated substrates were exposed to bacteria-inoculated medium, and the results showed that a large number of bacterial cells were present on the bare silicone substrate as well as the PDA and PEG-coated substrate.  These results indicated that the antibacterial properties of PDA and PEG coating were insufficient to prevent bacterial attachment and biofilm formation.  In the case of PEG-cationic polycarbonate coating, there were almost no bacterial cells on the surface after 7 days of exposure.  This result revealed the antibacterial property of the PEG-cationic polycarbonate coating due to a synergistic effect of hydrophilic PEG, which inhibits bacterial attachment, and the contact-killing properties of cationic polycarbonate, which kills bacteria.  Therefore, the PEG-cationic polycarbonate coating can effectively inhibit bacterial attachment and biofilm formation [130].
2.5.2.3 Adhesion Resistance-Biocide Leaching
The combined adhesion resistance-biocide leaching strategy has been used for antibacterial coating.  Ag-polytetrafluoroethylene (PTFE) was coated on SS substrate by electroless method.  Examination of the antibacterial properties of this coating revealed a decrease in the number of bacterial cells attached to the Agpolytetrafluoroethylene (PTFE)-coated substrate compared to the Ag-SS and SS substrates.  The superior antibacterial behavior of Ag-polytetrafluoroethylene (PTFE) might result from the synergistic leaching of Ag that kills bacteria and the adhesionresistance properties of hydrophobic polytetrafluoroethylene polymer.  Agpolytetrafluoroethylene (PTFE) also has good anticorrosive properties.  Therefore, an Ag-polytetrafluoroethylene (PTFE) coating layer may be suitable for MIC-inhibition applications [131].
A hydrophobic fluoro sol-gel coating was doped with silver ions for antibacterial purposes [132].  The coated and uncoated substrate was exposed to the bacteria-inoculated medium.  The results showed that a large number of bacterial cells adhered to the uncoated substrate, indicating that the uncoated substrate is a favorable surface for bacterial attachment and biofilm formation.  The number of bacterial cells was lower on the fluoro hybrid coating, potentially due to the fouling-release properties of this coating.  On the silver-doped fluoro hybrid coating, bacterial attachment was almost completely inhibited.  This might be due to the combined antibacterial properties of the hydrophobic fluoro hybrid coating, which lower bacterial adhesion to the surface, and the leaching of Ag ions that have biocidal properties [132].
The polymer coating was loaded with silver nanoparticle and top coated with PEG for antibacterial purposes [133].  The coated substrate was exposed to the bacteria inoculated medium and the results showed the higher biocide behavior for PEG-Ag NPs contained coating in compare of Ag NPs coating.  The release of silver ion could create inhibition zone, kill the bacteria and the PEG layer could inhibit the bacterial attachment on the surface [133].

2.6     Methods of Applying the Coatings  
There are generally different methods to immobilize antibacterial agents on the steel substrate.  These methods include (i) “grafting-to” strategies, involving immobilization of the antibacterial polymer on the substrate; (ii) “physical adsorption”, involving immobilization of antibacterial coating through non-covalent bonding; layer-by-layer coating is an example of this method; (iii) “grafting from” methods, involving the in-situ polymerization of the antibacterial polymer coating by covalent bonds with the substrate [134, 135].
It is not possible to obtain high polymer surface densities and thick coatings using “grafting-to” methods.  However, “grafting from” or “surface-initiated
polymerization (SIP)” is a suitable method to form a dense antibacterial polymer brush and obtain a good thickness [90, 95].
Generally, in grafting methods include “grafting-from” and “grafting-to” strategies, it is important to achieve strong adhesion between the antibacterial polymer brush and the metal substrate.  For this purpose, the steel substrate is initially functionalized with the anchor material.  The anchor material acts as an interlayer because it has strong affinity for the steel substrate and the polymer coating.  The presence of functional groups such as thiol, silane, and catechol causes strong bonding with steel, making the anchor material an interlayer that improves adhesion of the polymer to the metal substrate.  Different materials such as bromomethyl- or chloromethyl-terminated silanes, bromomethyl-terminated thiol, dopamine, and barnacle cement can be used as the anchoring material. The anchor agent may also act as a spacer that preserves the mobility of polymer brush and its biocidal properties.  The anchor material can also initiates polymerization of the monomer to obtain the antibacterial polymer.  For instance, bromomethyl- or chloromethyl-terminated silanes act as both anchor and initiator to form an antibacterial polymer on the substrate [90].  Recently, natural materials such as dopamine have been used as the anchor and initiator.  They have strong affinity for steel substrates due to the presence of catechol groups [136].
2.6.1     Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP)
One of the most studied and suitable methods to immobilize antibacterial polymers on the steel substrate to inhibit MIC are surface-initiated atom transfer radical polymerization (SI-ATRP), which is widely used in “graft-from” strategies [135, 137-139].  SI-ATRP is a controlled/living free-radical polymerization technique that enables the polymerization of a wide range of monomers and initiation from various types of substrates to produce well-defined functional polymer brush coatings [135].  In this method, the SI-ATRP initiator first covalently bonds to the steel substrate, and further polymerization occurs to form an antibacterial polymer.  SIATRP is mainly used to generate antibacterial polymer brushes with specific compositions, architectures, and functionalities on the substrate.
2.6.2     Other Coating Methods
Other methods such as covalent immobilization, electro polymerization, solgel, thermal curing and oxidative polymerization are used to apply coatings on the steel substrate for MIC inhibition.
The covalent-immobilization method is used to generate a chemical bond between the biocide agent and the substrate.  In this method, biocides such as QAS are tethered to the substrate or covalently bound to the matrix coating for antifouling applications.  The advantage of this method is that because the biocide agent is covalently bound to the substrate, it does not leach into the environment and is therefore environmentally friendly [140].
Electro-polymerization is also used to apply coating on the metal substrate for MIC inhibition applications.  This process is mainly used to deposit conductive polymers on metal substrates.  The electro-polymerization process is performed using a potentiostat/galvanostat.  In this method, the monomer is initially well dissolved in the acidic medium.  Then, polymerization is performed by applying current, and a thin layer of polymer is generated on the metal substrate.  By optimizing the electropolymerization time, monomer concentration, electrolyte solution, current and voltage values, an adherent protective coating can be obtained.  Electro-polymerization has several advantages: 1) no oxidants are required; 2) the polymer can be doped with different ions; 3) the thickness and morphology of the polymer film can be controlled [122].  The thickness of coating could be varied by different parameters such as monomer concentration, time of electro polymerization and the applied current.  Few microns to hundred microns could be the thickness of coatings.  The polymerization took place by formation of thin layer of coating on the substrate.
The sol-gel process is also used to apply coatings on metal substrates for antifouling and MIC inhibition purposes.  In this process, the liquid ‘sol’ becomes a solid ‘gel’; during transition, as the viscosity of the solution gradually increases, the sol is interconnected by cross-linking of the colloidal suspension.  This leaves a rigid porous gel with alcohol and water as by-products that gradually evaporate from the gel.  The sol-gel coating can inhibit MIC and biofouling in two ways: a) fouling release; in this method, the sol-gel coating is based on a hydrophobic component such as hybrid inorganic-organic silica-based coatings, which inhibit bacterial attachment and b) biocide leaching.  To effectively inhibit MIC and biofouling, the biocide agents are incorporated into the coating, which then leach out over time and kill bacteria.  The sol-gel method has several advantages including good-to-excellent performance against biofouling, low curing temperatures, enhanced and prolonged chemical and physical stability, ease of application, and the waterborne nature of sol-gel coatings [80, 104, 141, 142].
Thermal curing is also used to apply a coating layer on the metal substrate to inhibit MIC.  In this method, the antibacterial polymer is cured by increasing the temperature to specific values, and the coating is formed on the substrate.  The advantage of this method is that an adhesive polymer coating can be obtained on the substrate [143].
Oxidative polymerization is used to form different polymer coatings on the substrate.  In this method, the monomer of the related antibacterial polymer is initially attached to the substrate.  Then, using appropriate chemical materials, the monomer is converted to a polymer, forming a layer on the substrate [144].

2.7     Environmentally Friendly Coatings to Inhibit Microbial-Induced Corrosion
The application of effective environmentally friendly coatings to inhibit the MIC of steels is significant.  Different types of coatings such as self-healing coatings and silicon oxide based coating, have been used to inhibit MIC.  These coatings can inhibit bacterial attachment by contact killing or fouling release, and they are environmentally friendly because they only kill bacteria upon contact or minimize the surface energy, so the bacteria will not be able to attach to the surface.

2.7         Self-healing coatings
Self-healing coatings  are autonomic healing materials that respond without external intervention to environmental stimuli in a nonlinear and productive fashion, and have great potential for advanced engineering systems[17, 145]. Self-healing coatings, which autonomically repair and prevent corrosion of the underlying substrate, are of particular interest. Recent studies on self-healing polymers have demonstrated repair of bulk mechanical damage as well as dramatic increases in fatigue life[146, 147]. Various approaches for achieving healing functionality have been studied, including encapsulation [146, 148], reversible chemistry [18, 19, 149, 150], microvascular networks [151],nano-particle phase separation [20, 152, 153],polyionomers[154, 155],hollow fibers[21, 156],and monomer phase separation [145].
The majority of these systems, however, have serious chemical and mechanical limitations, preventing their use as coatings. Modern engineered coatings are highly optimized materials in which dramatic modifications of the coating chemistry are unlikely to be acceptable[157].
2.7.1        Micro/Nanocapsules embedment
Micro/Nanocapsules are uniformly distributed in the passive matrix keeping healing agent in a “trapped” state, thus avoiding the undesirable interaction between the active component and the passive matrix, leading to spontaneous leakage. As shown in figure 2.17, when the local environment undergoes changes or if the active surface is affected by outer impacts, the micro/nanocapsules respond to this signal and release encapsulated active material to heal the crack [158].

Figure 2.18: The autonomic healing concept. A micro/nanoencapsulated healing agent is embedded in a structural composite matrix containing a catalyst capable of polymerizing the healing agent [159].
2.7.2        Micro/Nanocapsules synthesis
Generally micro/nanocapsules are small particles containing a solid, droplet of liquid or gases as core material surrounded by a coating layer or a shell. Commercial microcapsules typically have a diameter 3µm –800µm, and consist of 10–90wt% core materials. They have been used for various engineering applications including carbonless copying papers, adhesives, cosmetics, insecticides and pharmaceutical materials[160]. Versatile core materials are encapsulated for several reasons, such as improvement of long-time efficiency, stabilization against environmental degradation, easy handling through solidification of liquid core, and maintenance of non-toxicity of degradation products[159].There are several methods to synthesis the microcapsules such as interfacial polymerization[161], coacervation    [162], in-situ polymerization[163],extrusion, and sol–gelmethods. But among these various methods, in-situ polymerization is the easiest and best process for encapsulation, because it does not require high level technology as shown in Figure 2.18.Hence most the researchers who worked on self-healing coatings have used in-situ polymerization as a major procedure for fabrication of micro/nanocapsules. The common healant material phase is liquid, because of free flow through the crack plane.
Figure 2.19: Comparison between the different encapsulation methods.

Generally micro/nanocapsules are small particles containing a solid, droplet of liquid or gases as core material surrounded by a coating layer or a shell. Commercial microcapsules typically have a diameter 3–800µm, and consist of 10–90 wt% core materials. They have been used for various engineering applications including [160] 9carbonless and pharmaceutical materials. Core materials are encapsulated for several reasons, such as improvement of long time efficiency, stabilization against environmental degradation, easy handling through solidification of liquid core, and maintenance of non-toxicity of degradation products [164].

Brown et al[163]  demonstrated an applicable procedure based on in situ-polymerization to encapsulate dicyclopentadiene (DCPD)as a healing agent with urea–formaldehyde (UF) shell. Microcapsules of 10–1000 µm in diameter were produced by appropriate selection of agitation rate in the range of 200–2000rpm in this procedure. This study is the main reference which the other scientists used with or without modification for micro/nanocapsules synthesis[163], but the in-situ polymerization method which was demonstrated by Brown et al. cannot produce micro-capsules finer than 10µm. Recently the process was modified by sonication to achieve submicron (nano) capsules[165]. A schematic diagram of this process is shown in figure 2.19. Sonication technique is transferring the ultrasonic energy from the probe to the solution medium with a specified time and energy[166].

Figure 2.20: Encapsulation method for preparing UF capsules containing DCPD using sonication. (a) Process flow chart; (b) Schematic showing the emulsion prior to sonication; (c) during sonication[165] After this modification, the final products were as fine as 600nm (Figure 2.20 (a)), which causes better compatibility with coating matrix. To improve the autonomous repairing ability of the system, various liquids as healing agent and their subsequent consequences were examined. The probability of encapsulating two types of resins and three different solvents as a binary system in the role of healing agent was investigated and the effects of the used healing agent on the shell thickness were found in another work [167].For achieving a thinner shell thickness and preventing capsules agglomeration, the researchers were forced to modify Brown’s method and the encapsulation shell wall materials and aqueous phase were decreased by half. Also capsules containing resin-solvent were fabricated by means of sonication and stabilization procedures .Figure 2.20 (b) shows the size distribution of synthesized capsules through this procedure which are all in the nano ranges.

Figure 2.21: Size distribution of synthesized capsules prepared (a) without sonication technique[168]. (b) with sonication technique[167].

2.7.3        Self-healing coatings as anti-fouling and corrosion resistance material
Self-healing micro-capsulated inhibitor incorporated in epoxy primer before painting on a steel surface was evaluated for its corrosion protection effectiveness on exposure to ASTM D 5894 electrolyte in laboratory and natural tropical sea-shore environment. The “healant” inhibitor was industrial custom-made and non-chromate organic-based microcapsules which were mixed into the primer before applying a polyurethane topcoat layer on steel surface. The results indicated that the active components in ruptured embedded inhibitor microcapsules were released into an inflicted scribe primer and topcoat film on steel surface on exposure to inhibit development of an electrochemical cell. Undamaged surface film of the test and control specimens exposed in the environments demonstrated excellent corrosion-inhibition performance as reflected by both visual inspection and electrochemical impedance spectroscopy experimental data. The results obtained on the performance of self-healing inhibitors should provide an understanding of the fundamental material property relationships of smart inhibitor coatings[169].
Figure 2.22: Optical micrograph of cross section of coating prepared in Experiment
Self-healing materials are a relatively modern and rapidly growing concept in materials science successfully introduced as an intended system feature in metallic materials [170], which try to create or to design man-made products able to fully or partially repair themselves upon external or internal damage and to recover the initial functionalities without the help of any external factors other than those derived from the damage-related environment itself [171] Anti-corrosion coatings systems based on encapsulating liquid healing agents, and on polymers and composites [159].

2.7.3        Zeolite, polyaniline and zeolite/polyaniline composite as anti-fouling and corrosion resistance materials.

Zeolite and polyaniline have recently been studied extensively for antifouling applications because of their unique properties such as low cost, ease of synthesis, environmental friendly and high environmental stability [172, 173].

In recent years, there has been renewed interest by researchers, packaging and coating industries to investigate novel types of safe and cost-effective antimicrobial materials; such as functionalized zeolites [174-177].

Zeolites play an important role in catalysis due to its micro-porous nature and shape selectivity. However, its application as electrochemical sensor in the detection of large molecules is limited due to the low surface area and microporous nature. To overcome this limitation, zeolites with interconnected intraor inter-crystalline mesoporosity have been developed [178, 179]. Several attempts have been made for the preparation of nano-sized zeolites using soft and hard templates [180-182]. Processed or natural zeolites can be used as protective coating materials from bacterial-induced corrosion [176].

Zeolites contain metal ions, such as calcium and sodium which are exchangeable by metals such as silver, copper, and zinc ions for use as antimicrobial agents [183-187]. The use of bio-toxic metals functionalized zeolites as antimicrobial agents in liquid media has been studied extensively [188-191]. It demonstrated was that zeolites exchanged with silver are highly hydrophilic and toxic to Escherichia coli, and reported that zeolite coating with a thickness of only 4–6 mm exhibited excellent adhesion to stainless steel and aluminium alloy for manned spacecraft application. Furthermore, the coatings were found to be extremely corrosion-resistant in strong acid and base solutions[190].

Polyaniline (PANI) with different morphology can be prepared without template. Morphology of the polyaniline can be tailored by varying the pH and organic additive [192]. Polyaniline exhibits good conductivity but its application as electrochemical sensor is limited because it does not contain any redox site. It was recently reported that redox site in the polyaniline can be incorporated by metal ion-exchange or coating a uniform layer of metal oxide on polyaniline matrix [193, 194].
The advantage of Polyaniline compared to conventional antibacterial agents is that these polymers only kill bacteria that come into contact with them and do not leach out; therefore, they are environmentally friendly [195]. The antibacterial effect of PANI in the dark against various types of bacteria was also examined.  This study revealed the high antibacterial activity of PANI in the dark, where conventional photocatalytic antibacterial agents cannot be used [196]. Electrostatic interactions between positively charged nitro-groups in the PANI chain and the negatively charged bacterial cell wall cause bacterial death [196].
A main disadvantage of PANI is that when exposed to the water medium, it loses its electrical conductivity due to the release of dopant ions.  The positively charged nitro groups become energetically neutral, and PANI is converted to its nonconductive form, which lacks antibacterial and antifouling properties.  To improve the electrical conductivity and antifouling properties of the coating, composites of PANI with other nanomaterials are generated [197].

Polyaniline exhibits good conductivity but has low surface area. Conventional zeolites exhibit low conductivity and porosity. It may be noted that nanocrystalline zeolites exhibit large surface area and large mesopore volume but lacks conductivity. To enhance the conductivity of zeolite, highly conductive materials are incorporated to make zeolite applicable for electro-catalytic applications [198]. It is reported in the literature that PANI-zeolite composites exhibit maximum conductivity with weight ratio of one [199].

2.8        Physical vapor deposition coatings (PVD)
PVD are atomistic deposition processes in which material is vaporized from a solid or liquid source in the form of atoms or molecules, transported in the form of vapor through a vacuum or low pressure gaseous (or plasma) environment to the substrate where it condenses [200]. PVD method is only involves only physical process such as plasma sputter bombardment or temperature vacuum evaporation rather than using chemical  reaction at the material surface as in Chemical Vapor Deposition (CVD). Usually PVD processes are used to deposit thin films with thickness around range of a few nano-meters to thousands of nanometers. Thin film is only applied to a layer which has thickness around of several microns or less (1 micron = 10-6 meters) and thicker than that it is called as coatings. There are three types of PVD coating that is vacuum evaporation, sputter deposition and ion plating. Each of the PVD utilizes the same three fundamental steps to develop coating [201]. The main of coating is actually a process where covering to a certain surface of substrate. This is done so that it will improve the surface properties of the substrate either for their appearance, corrosion resistance, wear resistance, adhesion purpose or others; figure 2.16 shows the schematic of the deposition chamber for RF magnetron sputtering technique.
Figure 2.16: Schematic of the deposition chamber for RF magnetron sputtering technique[201].

2.8.1        Vacuum evaporation
Vacuum evaporation is a process where a material from a thermal vaporization source reaches the substrate with a little or with no collision with gas molecules in the space between the source and substrate. The trajectory of the vaporized material is “line-of-sight”. The vacuum environment also provides the ability to reduce gaseous contamination in the deposition system to a low level. The thermal vaporization rate can be very high compared to other vaporization methods. Thermal evaporation is usually done using thermally heated sources such as tungsten wire coils or by high energy electron beam heating of the source material itself. Generally the substrates are mounted at an appropriate distance away from the evaporation source to reduce radiant heating of the substrate by the vaporization source. This method mostly used in applications such as mirror coatings, decorative coatings, electrical conducting films, wear resistant coatings and corrosion protective coatings.
2.8.2        Sputter Deposition
A deposition of particles vaporized from a surface of target by the physical sputtering process. Physical sputtering is a non-thermal vaporization process where the surface atoms are physically ejected from a solid surface by momentum transfer from an atomic sized energetic bombarding particle which is usually a gaseous ion accelerated from a plasma. Sputter deposition can be performed by energetic ion bombardment of a solid surface (sputtering target) in a vacuum using an ion gun or low pressure plasma where the sputtered particles suffer few or no gas phase collisions in the space between the target and the substrate. The sputtering source can be an element, alloy, mixture or and a compound and the material is vaporized with the bulk composition of the target. The sputtering target provides a long lived vaporization source that can be mounted so as to vaporize in any direction. Sputter deposition is widely used to deposit thin film metallization on semiconductor material, coatings on architectural glass, reflective coatings on compact discs, magnetic films, dry film lubricants and decorative coatings  [200].

2.8.3        Ion Plating
This method is utilizes concurrent of periodic bombardment of the depositing film by atomic-sized energetic particles, to modify and control the properties of the depositing film. In ion plating the energy, flux and mass of the bombarding species along with the ratio of bombarding particles to depositing are important processing variables. The energetic particles used for bombardment are usually ions of an inert or reactive gas, or in some cases, ions of the condensing film material. Ion plating can be done in a plasma environment where ions for bombardment are extracted from the plasma or it may be done in a vacuum environment where ions for bombardment are formed in a separate ‘ion gun’. By using a reactive gas in the plasma, films of compound materials can be deposited. Ion plating can be used to deposit hard coatings of compound materials, adherent metal coatings, optical coatings with high densities and
The selection criteria for determining the best method of PVD is dependent on several factors [202].

  1.  The type of material to be deposited
  2.  Rate of deposition
  3.  Limitations imposed by the substrate, such as, the maximum deposition temperature, size and shape.
  4.  Adhesion of the deposition to the substrate
  5.  Throwing power (rate and thickness distribution of the deposition process. Example: the higher the throwing power, the better the process ability to coat irregularly-shaped objects with uniform thickness)
  6.  Purity of coating materials
  7.  Equipment requirements and their availability
  8.  Cost
  9.  Ecological considerations
  10. Abundance of deposition material among the three section of PVD method there are several options of process that can be choose from. Some of the examples:

 

  1. a) Cathodic Arc Deposition

A technique that used an electric arc to vaporize a material from a cathode (target). The electric arc is striking in a high current but low voltage on the surface of the cathode. As a result a highly energetic emitted from an area called cathode spot. The temperature at the cathode spot is extremely high which makes a high velocity of vaporized cathode material leaving behind the cathode surface. The vaporized material then condenses onto the substrate thus forming a thin film. This technique can be applied to other types of material such as deposit metallic, composite and ceramic films. It is also known as Arc-PVD method. This method is applied in the semiconductor industry to grow electronic materials, in the aerospace industry to form barrier coatings to protect surfaces against corrosive environments [200].

  1. b) Electron Beam Physical Vapor Deposition.

Also known as EBPVD is a technique a target anode is heated to high vapor pressure and is bombarded with an electron beam causes atoms from the target to transform in to the gaseous state. These atoms then precipitate into solid form, coating everything that is in the vacuum chamber with a thin layer of the anode material. This method is suitable for industrial that need applications for wear resistant and thermal coatings such as in hard coating for cutting and tool industries, aerospace industries and optical films for semiconductor industries [200].

  1. c) Evaporative Deposition

A technique that in which a material is to be deposited is heated to a high vapor pressure by electrically resistive heating in low vacuum. The vacuum then allows the vapor particles to travel directly to the substrate where then it will condense back to the solid state. In this process evaporation takes two basic processes that are a hot source material evaporates and then condenses on the substrate. To deposit the material in good condition the substrate first must not be in rough surface. This is because the materials attack in a single direction thus if it is in a rough surface it will block the evaporated material from areas. This method is usually used in micro-fabrication to deposit metal films.

  1. d) Pulsed Laser Deposition (PLD)

Another method of depositing the thin film where a high power pulsed laser beam is focused in the vacuum chamber. This pulsed laser beam is then strike to the target of the material that needs to be deposited. The material then vaporized from the target which is deposit it as a thin film on a substrate. This technique can also occur in ultra high vacuum or in the presence of a background gas. Based from the steps given this process is just plain simple but actually its deposition techniques are quite complex.

  1. e) Sputter Deposition

This is a technique where a glow discharge bombards the material sputtering some away as vapor which then deposits onto the substrate. The advantage of this process is that even materials with high melting points are easily sputtered while evaporation of these materials in a resistance evaporator (Knudsen cell) is a problematic or even impossible. These techniques will mainly used in this project that is tried to deposit a thin film into the substrate that is mild steel [200].

2.8.4        Silicon Oxide as an Antifouling Coating

Although polymer-based antifouling coatings can be considered the best solution to date, there is still a space for improvement to make coatings much more lasting and suitable but not dangerous to humans and environment. Thus, silicon oxide is a suggested element for the new antifouling coating on the medium carbon steel substrate. This is mainly because of its unique characteristics of providing a smooth surface. Silicon oxide layers are easily grown on silicon or deposited on many substrates. Moreover silicon oxides are resistant to most chemicals used in silicon processing and yet can be easily patterned and selectively etched with specific chemicals or dry etched with plasmas. In addition, they can block the diffusion of dopants and many other unwanted impurities. Dopant in this case is a trace of impurity element that is inserted into a substance in order to alter the properties of substance. Silicon oxide also has high temperature stability (up to 1600°C) and for most the interface that forms between silicon and silicon oxide has very few mechanical or electrical defects and is stable over time [203].
Table 2.4 shows the common properties of silicon oxide.

Table 2.4: common properties of silicon oxide coating by PVD

Properties Value
Thermal Expansion Coefficient 5.6×10-7 /K
Density (thermal, dry/wet) 2.27-2.28 g/cm-3
Young Modulus 6.6x1010N/m2
Poisson’s Ratio 0.17
Thermal Conductivity 3.2×10-3 W/(cm.K)

According to Yingna, NiSi2 and NiSi were deposited on carbon steel by air plasma spraying. The result indicated that a protective SiO2 was formed on the surface of the coatings after oxidation. This would lead to degradation in mechanical properties of the substrate [204]. While according to Mittal, a thin film of MgFe2O4 was developed on Fe3O4 coated carbon-steel to improve the protectiveness of the film. As a result a better corrosion resistant product could be obtained. But a bi-layer oxide coating will be much expensive than one layer coating [205]. This is reason while in this research silicon oxide was proposed as a new coating as antifouling material that would perform better than the existent polymer coating in terms of adhesion, surface roughness and life time. The thickness of silicon films between 400 nm to 800 nm based on the parameters used [206] as shown in Figure 2.16

Figure 2.17: The cross-section of SEM pattern of the silicon film[206] 2.9     Summary
The literature reviews described the microbial-induced corrosion inhibition of steels using various coatings based on three main strategies: (a) biocide leaching (b) adhesion resistance (c) contact killing.  The MIC inhibition coatings based on biocide leaching strategy are mostly metallic which are toxic to the environment and have a cancerous effect on human.  The adhesion resistance coatings are not effective to inhibit biofilm formation and not suitable for MIC inhibition.  The coatings based on contact killing and fouling release strategies are mostly based on cationic polymers which have a good biocide behavior but they do not impart effective anticorrosive properties, thus they are not suitable for MIC inhibition.  There is a need to use an environmentally friendly coating that effectively inhibits MIC.  Self-healing and silicon oxide coatings are new and environmentally friendly type of coatings with both biocide and anticorrosive properties.  Thus, Self-healing and silicon oxide based coatings are good potential for MIC inhibition.  In this project, three different self-healing materials include zeolite, polyaniline, zeolite/polyaniline composite and silicon oxide were coated on mild steel substrate.
Zeolites play an important role in catalysis due to its micro-porous nature and shape selectivity. Processed or natural zeolites can be used as protective coating materials from bacterial-induced corrosion.
Polyaniline is used for MIC inhibition application because of its strong anticorrosive and good antibacterial properties.  Due to its high redox properties, PANI could passivate steel substrate impart anticorrosive properties. Due to presence of positively charged nitro-groups, PANI could display biocide behavior to kill the bacteria through contact killing strategy.
Polyaniline added to zeolite to form zeolite/PANI composite which exhibits maximum conductivity with weight ratio of one suitable for MIC induction.

Silicon oxide is a suggested element for the new antifouling coating on the medium carbon steel substrate. This is mainly because of its unique characteristics of providing a smooth surface. Silicon oxide layers are easily grown on silicon or deposited on many substrates. Moreover silicon oxides are resistant to most chemicals used in silicon processing and yet can be easily patterned and selectively etched with specific chemicals or dry etched with plasmas.

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